This invention relates to OFDM systems and, more particularly, to channel parameter estimation in OFDM systems that employ transmitter diversity.
In orthogonal frequency division multiplexing (OFDM) the channel is divided into many narrow subbands, which are transmitted in parallel, thereby, increasing the symbol duration and reducing or eliminating the intersymbol interference (ISI) caused by the dispersive Rayleigh fading environment. On the other hand, since the dispersive property of wireless channels causes frequency selective fading, there is higher error probability for those subbands with deep fading. Hence, techniques such as error correction code and diversity have to be used to compensate for the frequency selective fading of wireless channels. In this report, we investigate transmitter diversity using space-time codes for OFDM systems.
Recently, space-time coding has been developed for high data-rate wireless communications, and such coding has been extended to OFDM systems in a U.S. patent application titled xe2x80x9cOrthogonal Designs and Transmit Diversity for Wireless Communications,xe2x80x9d filed on Nov. 6, 1998 and bearing the Ser. No. 09/186,908. The space-time code is characterized by high code efficiency and good performance; hence, it is a promising technique to improve the efficiency and performance of OFDM systems. However, decoding of the space-time code requires the channel state information, which is a task that must be performed and, therefore, the efficiency with which the task is carried out is an issue.
Channel parameter estimation has been successfully used to improve performance of OFDM systems. For systems without co-channel interference, with estimated channel parameters, coherent demodulation is allowed, instead of differential demodulation, to achieve a 3-4 dB signal-to-noise ratio (SNR) gain. Moreover, for systems with receiver diversity, the maximum-ratio diversity combiner (MR-DC), which is equivalent to the minimum-mean-square-error diversity combiner (MMSE-DC) in this case, can be obtained using estimated channel parameters. For systems with co-channel interference, the coefficients for the MMSE-DC can be calculated from estimated channel parameters and instantaneous correlation of the signals from each receiver. However, no teachings can be found on the parameter estimation for OFDM systems with transmitter diversity.
Enhanced performance is obtained by estimating channel parameters during normal operation, in addition to an initial estimate based on a known training sequence, through use of the detected signals. Once the OFDM-modulated signals that are transmitted by the various transmitting antennas are detected, the signal received at a receiving antenna is employed to compute channel impulse response estimates between the signal received at that receiving antenna and the various transmitting antennas. The process is repeated for each receiving antenna.
In computing the channel impulse response estimates between the signal received at that receiving antenna and the various transmitting antennas, an nK0xc3x97nK0 matrix of terms (qxy[l]) is developed. Then, the inverse of the matrix is computed, and the computed matrix inverse is multiplied by a vector of terms (pi[l]), to obtain a vector of nK0-sample channel impulse response estimates. Each qxy[l] term is evaluated by selecting a signal sent by transmitting antenna x (i.e., bx[k], where k is an index designating a frequency subband of the OFDM transmitter), multiplying it by the conjugated signal of transmitting antenna y, i.e., by[k], further multiplying it by       ⅇ          (                        -          j                ⁢                              2            ⁢            π            ⁢                          xe2x80x83                        ⁢            kl                    K                    )        ,
and summing over all of the frequency subbands; all for impulse response sample time l. K is the number of frequency subbands. The pi[l] term corresponds to       ∑          k      =      1        K    ⁢            r      ⁡              [        k        ]              ⁢                  b        i        *            ⁡              [        k        ]              ⁢          ⅇ              (                              -            j                    ⁢                                    2              ⁢              π              ⁢                              xe2x80x83                            ⁢              kl                        K                          )            
where r[k] is the signal of the receiving antenna.